How the Hubble Space Telescope Found Evidence of an Ocean on Ganymede

There sure is a lot of wet stuff in our solar system. Ganymede, Jupiter’s largest moon, is the latest to show evidence of an ocean.

And, it has a lot of it. Scientists believe Ganymede’s subterranean ocean has more water than all the water on Earth’s surface.

John Grunsfeld, associate administrator of NASA’s Science Mission Directorate at NASA Headquarters described the evidence as “a significant milestone, highlighting what only Hubble can accomplish.”

“In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth,” Grunsfeld adds.

How did the Hubble Space Telescope do it?

Ganymede is the largest moon in our solar system and the only one with its own magnetic field. A magnetic field means aurorae, areas of hot electrified gas in regions near the north and south poles of the moon.

Ganymede also sits close enough to Jupiter to be affected by its magnetic field. This causes the aurorae to shift back and forth.

Joachim Saur of the University of Cologne in Germany led a team of scientists who pitched the idea of using the Hubble Space Telescope.

“I was always brainstorming how we could use a telescope in other ways,” said Saur. “Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior.”

If a massive saltwater ocean is lying beneath Ganymede’s surface, Jupiter’s magnetic field would create a second magnetic field in the ocean that would counter Jupiter’s.

That’s what scientists found. The ocean’s magnetic field reduces the shifting of the aurorae to 2 degrees. Without the ocean, it would be 6 degrees.

Using this, scientists are able to estimate how big the ocean is on Ganymede. And, it is big. Estimates put it at 60 miles thick. That’s 10 times deeper than Earth’s. It’s also buried under 95 miles of icy crust.

Hubble was the perfect telescope for the job. The newest observations were done in UV light. Our atmosphere does a great job blocking most of this. So, a telescope above the Earth’s atmosphere is needed.

The Hubble Space Telescope has given us nearly 25 years of incredible views of our solar system and beyond. On April 24, it will celebrate 25 years since its launch back in 1990. And, it still has a few more years left to go.

Hubble’s successor, the James Webb Space Telescope, is expected to launch in October 2018. One of JWST’s primary goals will be searching for the light from the first stars and galaxies. The telescope’s primary mirror will have a collecting area about five times larger than the Hubble’s.

Get ready for even more breathtaking space images starting in late 2018.